Heterojunction field effect transistors are known in the prior art, and high speed devices have two semiconductors of different band gaps arranged to produce a two-dimensional electron gas at the heterojunction interface. The electron gas enhances the operating speed of the device, and is produced by doping impurities into one of the two semiconductors forming the heterojunction which has the larger band gap, making that layer of n-type material.
An example of such a prior art heterojunction field effect transistor is illustrated in FIG. 3. In that figure, a channel layer 12 is formed of GaAs, and an electron supplying layer comprising n-type Al.sub.x Ga.sub.1-x As 11 is produced on the GaAs substrate. The heterojunction thus formed produces a two-dimensional electron gas 4 at the boundary between the channel layer 12 and the electron supplying layer 11. A source electrode 5, a gate electrode 6, and a drain electrode 7 are produced at the appropriate positions on the electron supplying layer 11, with appropriate connections formed to the electrodes for connection into a circuit.
FIG. 4 shows an energy band structure of the heterojunction field effect transistor of FIG. 3. It is seen that the n-type Al.sub.x Ga.sub.1-x As/GaAs heterojunction has a relatively large band gap .DELTA.E.sub.c which is desired in such a device in order for the electrons from the donor material in the layer 11 to produce an electron gas just below the Fermi level of the channel layer 12. If the energy band gap .DELTA.E.sub.c is too small, the two-dimensional electons will return to the conduction band of the donor or source material. Therefore, a larger band gap keeps the electrons in the two-dimensional electron gas where they can serve as carriers in the channel.
For the foregoing reasons, it has been conventional to use doped Al.sub.x Ga.sub.1-x As for the donor level and GaAs for the underlying undoped semiconductor layer, because the AlGaAs has a larger band gap than the GaAs. In such a structure, the n-type AlGaAs layer 11 functions as the electron supplying layer and the undoped GaAs layer functions as the channel layer.
In order to enhance the high frequency performance of a heterojunction field effect transistor in the same element size, it is worthwhile to reduce the gate to source parasitic resistance R.sub.s. The gate to source parasitic resistance R.sub.s of a heterojunction field transistor is represented by the sum of the source contact resistance R.sub.co and the gate to source channel resistance R.sub.sg. EQU R.sub.s =R.sub.co +R.sub.sg ( 1)
Since the source contact resistance R.sub.co is relatively small as compared to the gate to source channel resistance R.sub.sg, the contact resistance R.sub.co can be ignored and in order to reduce R.sub.s, attention is directed to reducing R.sub.sg. The gate to source channel resistance R.sub.sg is represented by the following expression: ##EQU1## where n.sub.s represents the two-dimensional electron concentration, .mu. represents the electron mobility, w.sub.gt represents the gate width, L.sub.sg represents the gate to source spacing, and q represents the elementary electrical charge.
It is therefore seen that in order to reduce R.sub.sg for a structure of a given size (i.e., L.sub.sg /W.sub.gt is constant) it is necessary to increase the two-dimensional electron concentration n.sub.s and/or the mobility .mu.. The two-dimensional electron concentration n.sub.s in the prior art device is approximately represented by the following expression: EQU n.sub.s .perspectiveto.(2.epsilon.N .DELTA.E.sub.c /q).sup.1/2( 3)
where .DELTA.E.sub.c represents the conduction band energy discontinuity between the n-type Al.sub.x Ga.sub.1-x As and the GaAs, N represents carrier concentration of the n-type AlGaAs and .epsilon. represents the permittivity of the n-type Al.sub.x Ga.sub.1-x As. Thus, it will be appreciated that in order to increase the two-dimensional electron concentration n.sub.s, it is necessary to increase the carrier concentration N or the conduction band energy discontinuity .DELTA.E.sub.c or both.
Focusing now on the mobility .mu. of the two-dimensional electron gas, such mobility is approximately represented by the following expression: ##EQU2## where .mu..sub.L is determined by the lattice vibration of the channel material in which the two-dimensional electron gas is produced, and generally increases with the decrease of the effective mass of the material at a constant temperature. The factor .mu..sub.i represents the mobility due to ionized impurity scattering, and this factor increases with the decrease in impurity concentration and with the decrease in Coulomb interaction at a constant temperature. Thus, .mu..sub.i is determined by the ion concentration in the non-doped GaAs and the Coulomb interaction between the two-dimensional electron gas and the ions in the n-type Al.sub.x Ga.sub.1-x As.
In attempting to reduce the gate to source channel resistance by increasing the carrier concentration of the AlGaAs layer in the prior art structure described above, at least two major problems arise. First of all, Si doped into the AlGaAs layer also acts as the so-called DX centers as well as shallow donors. The shallow donors can supply electrons into the interface of the GaAs/AlGaAs, however, the DX centers cannot because the DX centers are located farther from the conduction band. Therefore the carrier concentration in the AlGaAs layer cannot increase proportionately with the Si doping level. Secondly, the conduction band discontinuity .DELTA.E.sub.c increases with the increase in the A1 composition proportion x of the Al.sub.x Ga.sub.1-x As. When the x increases, however, the proportion of donors which function as DX centers and the activation energy of the donors also increase, and the carrier concentration N is reduced at the same doping level. Thus, it is not possible to increase only the .DELTA.E.sub.c independently, and at some value of conduction band discontinuity .DELTA.E.sub.c the product of carrier concentration N and conduction band discontinuity .DELTA.E.sub.c reaches the maximum at the same doping level. Usually, the maximum .DELTA.E.sub.c is about 0.3 eV. When .DELTA.E.sub.c is about 0.3 eV, the maximum value of the shallow donors doped into the Al.sub.x Ga.sub.1-x As layer is about 1.times.10.sup.18 cm.sup.-3, and then the maximum two-dimensional electron concentration n.sub.s determined from the expression 3 is about 2.times.10.sup.12 cm.sup.-2.
For the foregoing reasons, in dealing with the prior art heterojunction field effect transistor it was not possible to further reduce the parasitic resistance and therefore increase the high frequency performance.